(1) Field of the Invention
The invention relates to a transmission system for the transmission of a bi-phase modulated signal which is divided into T sec. intervals, the waveform in an interval being given by one period of a carrier having the frequency 1/T or of a carrier whose phase is shifted 180.degree. relative thereto, depending on the modulating binary data signal.
The power spectrum of such a bi-phase modulated signal has zero points at 0 Hz and at 2/T and a maximum between 0 Hz and 1/T in the region of 1/T. Bi-phase modulation is inter alia used in cases in which a second signal must be transmitted in addition to a baseband signal. At low frequencies the bi-phase signal contains little energy, as a result of which the transmission of an independent baseband signal is possible at these frequencies.
(2) Description of the Prior Art
By means of bi-phase modulation of the type mentioned here the transmission rate is doubled compared with the original data signal, which is accompanied by an increase of the frequency band which in principle extends from 0 Hz to 2/T. The frequency 1/T is equal to the symbol frequency of the modulating signal and is designated as the bit frequency. The bi-phase modulated signal covers a frequency band having a width of approximately twice the bit frequency.
Generally, a receiving filter for bi-phase modulated signals will be provided by a Nyquist-I filter having a frequency response which continues to twice the bit frequency (passband) and which is zero thereabove (stopband). The transfer function H(w) of such a filter for pulse-shaped signals is, for example, given by: ##EQU1## (w)=2.pi.f is the angular frequency for .vertline.w.vertline.&gt;4.pi./T.
In the event of white noise such a filter results for bi-phase modulated signals in a signal-to-noise ratio (S/N) which is approximately 9 dB lower than for unmodulated NRZ-signals.